1. Field of the Invention
The present invention relates to a device for measuring the intensity of the light scattered by high concentrations of particles or macromolecules ranging between a plurality of nanometers and hundreds of microns. It is more particularly applied to photon correlation.
The present invention is more particularly directed to measuring the intensity of the light scattered by thin films of colloidal media.
2. Description of the Prior Art
Particle size measurements by quasi-elastic light scattering or photon correlation have been widely used for about 20 years. Particles or macromolecules of a diameter ranging between a plurality of nanometers and a plurality of microns may be characterized by this method which depends on RAYLEIGH's scattering theory.
To that effect, objects are introduced in a liquid (solvent or not) in a very low proportion. The prepared sample is introduced into a measuring cell which is traversed by a focused laser beam. The Brownian movement of the particles produces variations in time of the scattered light. These variations are directly proportional to the size of the particles.
The use of a suitable optical detector (spectrodiffusiometer, spectrogoniometer, etc.), a correlator and mathematical signal processing methods (Laplacian inversion, Fourier transform) thus allows a frequency distribution and therefore a size distribution of the particles to be known.
The theoretical analysis rests on the known hypothesis that each particle acts as an individual diffuser and there is no interactive effect between the particles, i.e. there is no "multiple scattering" which might limit or even prevent any interpretation of the variations of the observed scattered light.
Conventional submicronic grain-size analyses by photon correlation thus only analyze very low particle concentrations.
In order to remedy this drawback, a known solution is to introduce an optical fiber into the medium. The particles scatter light emitted at the end of the optical fiber. The light backscattered by the particles goes back through the optical fiber and reaches the photosensitive detector via a directional separator in order to prevent the source light from reaching the photodetector.
However, a certain amount of light is directly reflected from the fiber even before it comes out of it and reaches the detector. It must therefore be measured as a reference signal (heterodyne system).
Other means have been investigated, in the Field of backscattering with conventional cells. But, the reflection effects of the incident beam and the strong multiple scattering have not allowed useful results to be obtained. The conventional instruments for characterizing submicron particles by photon correlation as illustrated in FIG. 1 include a laser generator (not illustrated) generating a rectilinear laser beam 2. A cylindrical vessel or a vessel with parallel faces 3 containing a mixture of objects, particles or macromolecules (M) is arranged on the path of laser beam 2. The laser beam 2 is focused by means of a converging lens 4 on the center of this vessel. The objects (M) may be in solution or in suspension in the liquid. A detector 5 including an optical system and a photosensitive unit receives scattered light from the center of the vessel at an angle with respect to the axis XY. The photosensitive unit is then connected to a signal processing computer (not illustrated). The objects (M) are illuminated by scattered light from the beam 2. The Brownian movement of the objects produces variations in time of the scattered light. These variations are directly linked to the size of the objects and they may be studied, for example, by means of an analysis of the photon correlation spectroscopy type. In FIG. 2, it is easy to understand why this known device does not allow analysis of concentrated samples. In fact, as detector 5 observes a solution of objects (M) at an angle with respect to axis XY greater than 90.degree. (backscattering), one receives, in addition to the characteristic direct scattering signal 6, the luminous rays due to the multiple scattering 7, and the rays from the reflection of laser 8 entering vessel 3. These latter parasitic luminous rays create a poor signal-to-noise ratio which makes it impossible to study the low variations cited above.